Biology Reference
In-Depth Information
The role of RAD54 in stimulating RAD51-mediated three-strand exchange
activity and D-loop formation was first demonstrated with recombinant yeast
proteins.
283
This stimulation is seen with many RAD54 orthologs and occurs in a
species-specific manner.
277,284-286
Sub-stoichiometric amounts of RAD54 are
sufficient to greatly stimulate the RAD51 recombinase activity
in vitro
,
283,284,286
indicating that the protein functions in a catalytic manner. In
fact, for RAD51-mediated strand exchange stimulation, the ATPase activity of
RAD54 is required.
264
Conversely, RAD51 improves the ATP hydrolysis and
translocation ability of RAD54 on dsDNA.
284,287
RAD54 facilitates dissociation of RAD51 from heteroduplex DNA follow-
ing strand exchange in an ATP-dependent manner.
198,288
Overexpression of
Rad51 in Rdh54-deficient background leads to arrest of cell growth in yeast
due to accumulation of Rad51 on undamaged chromatin.
197
Furthermore, it
was revealed that Rad54 is specialized for removal of Rad51 from damage-
induced foci, while Rdh54 is involved in disassembly of Rad51 from unda-
maged toxic dead-end dsDNA complexes.
197
During the postsynaptic phases, Rad54 enhances the heteroduplex extension
of Rad51.
289
Human RAD54 has a relatively higher affinity for Holliday junctions
and PX junctions compared to dsDNA, and it exhibits branch migration activity
in a multimeric functional complex.
276,290
Furthermore, budding yeast Rad54
and human RAD54 have been shown to physically interact with the Holliday
junction resolvase Mus81-Mms4 and MUS81-EME1, respectively, to stimulate
Holliday junction resolution.
291,292
Even though the branch migration activity of
RAD54 was not required for MUS81-EME1 stimulation, ATP was required.
292
D. RAD51AP1
RAD51AP1 (RAD51-associated protein 1), previously known as PIR5, en-
hances RAD51-mediated joint molecule formation by physically interacting with
both RAD51 and joint DNA structures and dsDNA molecules.
293-295
Knock-
down of RAD51AP1 in human cells increases genotoxic stress to DSB-inducing
agents.
293,294
However, the detailed role of RAD51AP in RR remains a mystery.
VI. DSB Repair in Chromatin
A. DSB-Induced Histone Modifications
In higher eukaryotes, DNA is compacted into chromatin and the basic unit
is a nucleosome, which consists of 146 bp of DNA wrapped approximately 1.7
times in left-handed superhelical turns around a tetramer of histones H3 and
H4 with two H2A-H2B dimers.
296
There are several levels of compaction of
chromatin
in vivo
.
296,297
The basic nucleosome structure is arranged into an
